Early power door lock systems using an electric motor to power the lock lift arm up and down faced the problem of difficult manual operation. That is, if the lock lift arm was directly mechanically coupled to the motor, manual operation thereof would have to overcome the resistance of the motor. Several different clutches have been used, and many more proposed, for coupling the motor to the lock lift arm when the motor starts, but decoupling it when it stops.
Examples abound of various approaches to decoupling. An early clutch shown in U.S. Pat. No. 3,947,060 used a simple frictional slip device consisting of a rubber ring and hub which would turn one to one when turned slowly, by the motor, but which would slip when turned quickly by a manual override handle. U.S. Pat. No. 4,706,512 shows a rotary load sensitive releasable coupling consisting of a spring loaded wedged shaped tooth that cams past a stop at the end of travel of the power actuator so that later manual operation will not have to back drive the motor. U.S. Pat. No. 4,674,781 uses a lost motion coupling and a spring to drive a rack back to neutral position at the end of stroke so that manual operation will not see the resistance of the motor or the gearing. U.S. Pat. No. 4,573,723 uses a complex series of gears, wave washers and swing levers to reverse one of the gears when the motor stops and decouple the motor from the lock lift arm.
Despite the myriad possible approaches above, the more common approach is to use some type of centrifugal clutch interposed between the motor and whatever gear system is used to directly shift the lock lift arm up and down. Typically, a weighted member that is radially expandable outwardly under centrifugal force is located inside a cupped output member. The drive shaft is connected to the weighted member, and the cupped output member is somehow drivingly engaged to the lock lift arm. The weighted member is contracted and in its stationary state, but expands outwardly when spun to engage and drive the cup and the lift arm. When the motor is stopped, the weighted member contracts again, so that manual operation of the lock lift arm will not have to back drive the motor.
Many different designs for such a centrifugal clutch have been used and proposed. Most have some objectionable shortcomings in terms of complexity and consequent difficulties in assembly, operation, and cost. U.S. Pat. No. 4,311,331 shows a very complex centrifugal clutch with several weights, pivot pins and return springs. U.S. Pat. No. 4,610,343 discloses a clutch of nearly equal complexity, one which has the added feature of a series of elastic pillars located in the spaces between a central driver and a series of peripheral weighted shoes. The pillars are designed to wedge between the driver and the shoes when the shoes have made contact with the cupped output member, so as to increase the contact force therewith. Some systems go farther and also decouple the lock lift arm from the gear drive mechanism that runs the lock lift arm, to remove that resistance as well from manual operation. An example may be seen in U.S. Pat. No. 4,093,289. The centrifugal clutch disclosed there FIGS. 2 and 3 shows a series of weights on slides, but no means is disclosed, apparently, that would cause the weights to retract when stationary.